Deep UV Photon-Counting Detectors and Applications

Transcription

1 Invited Paper Deep UV Photon-Counting Detectors and Applications Gary A. Shaw, Andrew M. Siegel, Joshua Model, Adam Geboff MIT Lincoln Laboratory, Lexington, MA Stanislav Soloviev, Alexey Vert, Peter Sandvik GE Global Research, 1 Research Circle, Niskayuna, NY ABSTRACT Photon counting detectors are used in many diverse applications and are well-suited to situations in which a weak signal is present in a relatively benign background. Examples of successful system applications of photon-counting detectors include ladar, bio-aerosol detection, communication, and low-light imaging. A variety of practical photon-counting detectors have been developed employing materials and technologies that cover the waveband from deep ultraviolet (UV) to the near-infrared. However, until recently, photoemissive detectors (photomultiplier tubes (PMTs) and their variants) were the only viable technology for photon-counting in the deep UV region of the spectrum. While PMTs exhibit extremely low dark count rates and large active area, they have other characteristics which make them unsuitable for certain applications. The characteristics and performance limitations of PMTs that prevent their use in some applications include bandwidth limitations, high bias voltages, sensitivity to magnetic fields, low quantum efficiency, large volume and high cost. Recently, DARPA has initiated a program called Deep UV Avalanche Photodiode (DUVAP) to develop semiconductor alternatives to PMTs for use in the deep UV. The higher quantum efficiency of Geiger-mode avalanche photodiode (GM-APD) detectors and the ability to fabricate arrays of individually-addressable detectors will open up new applications in the deep UV. In this paper, we discuss the system design trades that must be considered in order to successfully replace low-dark count, large-area PMTs with high-dark count, small-area GM-APD detectors. We also discuss applications that will be enabled by the successful development of deep UV GM-APD arrays, and we present preliminary performance data for recently fabricated silicon carbide GM-APD arrays. 1. INTRODUCTION At an ultraviolet wavelength of 250nm, a source with just 1mW of optical output power produces over 1,000,000 billion (10 15 ) photons per second. While this number seems unfathomably large, there are in fact many applications in which detecting and time-tagging the arrival of individual photons is desirable. The ultraviolet (UV) region of the electromagnetic spectrum refers to light with wavelengths ranging from about 10nm up to 400nm. The UV spectrum is further subdivided into regions or subbands, some of them overlapping, as summarized in Table 1. In the literature, the boundaries of these regions are not consistently defined, and other names may be used to refer to the subbands. For example, the term deep-uv is often used to refer to the UV-C band. Table 1: Ultraviolet spectral region and subband definitions Band Wavelength Photon Name of Ultraviolet Subband Abbreviation Range (nm) Energy (ev) Ultraviolet A, long wave, or black light UVA Near NUV Ultraviolet B or medium wave UVB Middle MUV Ultraviolet C, short wave, or germicidal UVC Far FUV Vacuum VUV Extreme EUV Communication: MIT Lincoln Laboratory, 244 Wood Street, Lexington, MA Advanced Photon Counting Techniques III, edited by Mark A. Itzler, Joe C. Campbell, Proc. of SPIE Vol. 7320, 73200J 2009 SPIE CCC code: X/09/$18 doi: / Proc. of SPIE Vol J-1

2 There are a number of diverse reasons for interest in deep-uv photon-counting detectors. One advantage to optical systems that operate at short wavelengths is that finer diffraction-limited resolution that can be achieved for a given size aperture. Another advantage is the absence of solar background in the deep-uv spectral band. At wavelengths from about 280 down to 200nm, termed the solar-blind region of the UV spectrum, ozone and oxygen in the atmosphere absorb all of the UV radiation from the sun, resulting in benign backgrounds even in full mid-day sun. However, in order to take advantage of the low backgrounds in this region, the detector must either be totally unresponsive to light from the near-uv and visible portion of the solar spectrum (solar-blind), or the near-uv and visible background flux must be suppressed by appropriately designed filters. In practice, it is difficult to design and fabricate UV detectors that are totally unresponsive to longer wavelength background in the near-uv and visible, so an appropriately designed filter is generally required, even with so-called solar-blind detectors. The same phenomena that are responsible for the existence of benign backgrounds, namely strong absorption and scattering of UV-C solar illumination by the atmosphere, are also an impediment to applications requiring active illumination such as stand-off ladar. 2. PHOTON COUNTING DETECTORS A variety of device technologies have been employed to realize photon-counting detectors. Not all technologies function in the deep-uv and of those that do, there are significant performance issues to consider when selecting a detector technology for a particular application. In this section, system-level performance metrics applicable to photoncounting detector technology are identified and then employed as the basis for comparing performance of both commercial off-the-shelf (COTS) and emerging research-grade detectors System-Level Detector Performance Metrics In considering the various detector technologies that might be employed in a particular deep-uv photon-counting application, the performance measures most frequently of interest include the efficiency of the detector in converting photons to detectable current pulses, the internally-generated false count ( dark count ) rate, and the maximum useable counting rate, or bandwidth, of the detector. A parameter related to detection bandwidth is the temporal stability with which the arrival of a photon can be measured, called timing jitter. Depending upon detector specifics, the photon conversion efficiency may be quoted either as quantum efficiency (QE) or as single photon detection efficiency (SPDE). The mechanisms responsible for the generation of dark counts also vary with the detector technology and design. As a rule of thumb, the dark count rate grows in proportion to the active area of the detector, so comparisons of dark count rate performance across detectors should always be normalized by the active area of the detector. For the reasons discussed in Section 1, the responsivity or QE of the detector at longer wavelengths (out-of-band) may be an important factor in determining the utility of a particular detector technology for deep-uv, especially when the application involves solar backgrounds. Even when the performance metrics mentioned thus far are suitable for an intended application, pragmatic considerations of size, weight, power consumption, sensitivity to environmental extremes, and cost may eliminate a particular detector technology from consideration Commercial Detector Technology The two broad classes of photon-counting detectors addressed in this paper are PMTs and Geiger-mode APDs (GM- APDs). Within each of these classes, design and material variations lead to families of devices with wavelengthdependent performance differences. For simplicity, we identify a single commercial device within each class as a baseline against which to compare the performance of emerging wide-band gap GM-APD devices. There is no implied endorsement in our particular selection of representative commercial detectors. Proc. of SPIE Vol J-2

3 Photomultiplier tubes Channel photomultipliers (CPM) are a specific class of PMT that exhibit a low dark count density, good discrimination performance, and commercial availability. The Perkin Elmer MP-1921 channel photomultiplier module [1] (CPM) includes an internal high-voltage bias supply and discrimination circuit that delivers a digital pulse for each photon detected. The MP-1921 has a large active area (1.9cm 2 ), and an extremely low dark count rate (<10 cps). Furthermore, the CPM employs a cesium telluride (CsTe) photoemissive cathode resulting in a high degree of solar rejection, since the quantum efficiency rolls off steeply at around 300nm. Photos of the CPM and an associated quantum efficiency curve are shown in Figure 1. The 16mm CPM head for the module, shown in Figure 1c, only consumes about 45mm 3 of the total packaged module volume. The remaining 280mm 3 are devoted to the high-voltage bias supply, discriminator circuitry and RS-232 interface. Quantum Efficiency (%) cm High Voltage Supply 13 cm 5 cm 10 Photomultiplier Tube Wavelength (nm) 82mm PMT 16mm (a) (b) (c) Figure 1: (a) Perkin Elmer channel MP-series photomultiplier module (b) quantum efficiency as a function of wavelength (c) interior view of module showing high-voltage power supply, photomultiplier tube, and interface wiring Geiger-mode APDs Silicon GM-APDs are relatively mature. When operated in Geiger-mode to count individual photon arrivals, the avalanche current must be quenched, either actively or passively, prior to re-arming the APD. Three increasingly more complicated quenching schemes are illustrated in Figure 2. V a V OUT (t) V a Quenching Driver R L R L h h h R S R S -V a (a) (b) (c) Figure 2: Three GM-APD quenching schemes (a) asynchronous passive quenching, with reset time dominated by the R L C J time constant. (b) Gated quenching, with an enable pulse prebiasing the APD prior to an event of interest. (c) asynchronous active quenching, with the reset time determined by the quenching driver design. Proc. of SPIE Vol J-3

4 C0 During the time between firing of the APD and resetting, the APD is essentially blind to the arrival of additional photons. This dead time can be modeled as an average loss in sensitivity, referred to as a blocking loss. The speed with which the re-arming operation can occur is determined by the RC time constant of the bias circuit in passive quenching mode, or by the design of the quenching circuit in active-quenching mode. In order to achieve fast recovery of the GM- APD, low capacitance, and thus small active areas are desirable. As the active area of the APD increases, the capacitance and dark count rate tend to grow in proportion Pixellated Geiger-mode APD Pixellated GM-APD arrays, consisting of many individual passively quenched APDs connected in parallel, were designed to overcome the high blocking loss of a single large-area GM-APD detector. Silicon pixellated GM-APDs are commercially available from several sources [2]-[3]. For the purpose of photon-counting in the deep-uv, not all Si GM- APD device designs are equally effective. UV-C single photon energy levels are significantly higher than the band-gap of silicon (> 4.5eV versus 1.12 ev) so photon absorption tends to occur near the surface of the APD. Consequently, APD designs can be optimized for better performance in the UV-C band. Hamamatsu Corporation has a family of Si GM-APD arrays with SPDEs comparable to or better than the QE of CsTe PMTs in the UV-C band. Figure 3 shows a Hamamatsu 1mm x 1mm Multi-Pixel Photon Counter (MPPC) along with an illustration of the method of parallel connection of the individual APDs employed in the MPPC device. Quenching Resistor U0-20 C 30 1 of N GM-APD Pixels WOVELENCTH (rcnc) (a) (b) (c) Figure 3: Hamamatsu Multiple Pixel Photon Counting (MPPC) array (a) 50 m pixel detector (b) parallel connection of individual GM-APD detectors comprising the array (c) manufacture s nominal PDE as a function of pixel sizes. The MPPC device is available with 3 different pixel sizes (25 m, 50 m, 100 m) and three different physical sizes (1mmx1mm, 2mmx2mm, 3mmx3mm). As shown in Table 2, for the 1mmx1mm part, the choice of pixel size primarily affects the SPDE (in part due to fill factor), the aggregate array dark count, and the recovery time (which limits the maximum achievable bandwidth). Table 2: Hamamatsu specifications for passively-quenched MPPC GM-APD arrays Parameter Hamamatsu S Series Device Suffix -025C -050C -100C Units Physical area 1x1 1x1 1x1 mm Number pixels 40x40 20x20 10x10 Pixel size m Fill factor % SPDE at 400nm % SPDE at 270nm % Dark count rate khz Recovery time ns Proc. of SPIE Vol J-4

5 While photon-counting in the deep-uv can be accomplished with silicon GM-APDs, the fact that silicon responsivity peaks in the visible, as shown in Figure 3c, means that effective blocking filters for suppression of out-of-band (e.g. near-uv and visible) flux are difficult to design and fabricate. All of the pixels in commercial pixellated GM-APDs are connected in parallel. This parallel connection architecture results in a sequence of charge pulses that, over the linear region of the APD (i.e., small signal, low blocking loss regime,) is proportional to the number of simultaneous photon detections at any instant. However, the fact that the APDs are not individually addressable or controllable precludes use of MPPC arrays for imaging applications. 3. SOLAR-BLIND GEIGER-MODE AVALANCHE PHOTODIODE DETECTOR ARRAYS As indicated in the previous section, CsTe photomultiplier tubes are solar-blind detectors suitable for many deep-uv sensing applications with large active area, low dark count,. However, the bandwidth of PMTs is too low for some applications, and their size and cost precludes the development of arrays for small form-factor imaging applications. Commercially available Si APDs demonstrate moderate quantum efficiencies in the ultraviolet and solar-blind range, but require expensive optical filters to achieve a high solar photon rejection ratio with wide field of view as their response extends through the visible wavelength range. Si-based APDs with very low dark counts were demonstrated when they were cooled to liquid nitrogen temperatures [4]. However, typical dark current densities of these devices at a gain of more than 10 are on the order of ~100 na/cm 2 at room temperature, which makes them hard to operate in a single-photon detection mode without cooling. In addition, the electronic properties of silicon impose an upper limit on the operating temperature of silicon-based electronic devices at roughly 200 C for devices fabricated on conventional bulk silicon substrates due to rapidly increasing leakage current. While Si detectors and PMTs are often detectors of choice, novel GaN-based detectors have seen advancements recently. GaN-based APDs were demonstrated with a high sensitivity in the solar-blind region and gain of more than 1000 [5]-[7]. Reported dark currents at a gain of 1000 were typically more than 100 na/cm 2 for these devices, likely due to high defect densities present in the GaN material. SiC-based APDs have been demonstrated with high quantum efficiencies in the solar-blind wavelength range, and have been shown to have high gain and low dark currents [8]-[12]. Recently reported devices with dark current densities of 63 na/cm 2 demonstrated a gain of more than 10,000 and also showed a very low noise equivalent power of 20 fw [13]. Typical SiC-based photo detectors show response in the range from 200 to 400 nm with a peak value between 270 and 300 nm. For solar-blind applications, in order to limit the SiC APD response to the solar-blind wavelengths, an optical filter may be used to reject the light above approximately 280 nm, where some light absorption occurs due to indirect nature of SiC band structure Silicon Carbide GM-APD Array Development APD epitaxial structures with separate absorption and multiplication (SAM) regions were designed and optimized. Typical SAM-APDs are designed to achieve reach-through (when an electric field in the absorption layer reaches N+ cap layer) prior to reaching high gain in order to obtain fast response. This enhances the likelihood that photogenerated carriers in the absorption region will be swept into the multiplication region and contribute to photocurrent. However, the appreciable electric field in the absorption region increases the overall dark current and thereby decreases the signalto-noise ratio of the detector. The 4H-SiC SAM-APDs presented here were intentionally designed to operate under nonreach-through conditions. A schematic cross-sectional view of the 4H-SiC SAM-APD is shown in Figure 4, while the 2-D distribution of electric field at an avalanche breakdown voltage simulated with SILVACO model is shown in Figure 5. Note, that maximum electric field is uniformly distributed in a bulk of the device (not at the edges) due to a positive bevel structure. Using n- type doped materials for the absorption and charge layer was beneficial as the diffusion length of minority holes in n- type 4H-SiC is significantly longer than that of minority electrons in p-type. This results in the diffusion of Proc. of SPIE Vol J-5

6 comparatively more carriers into the avalanche region which give rise to higher photocurrents particularly at higher energies where the photon penetration depth is small PI,'trrc Field (Vic n-4h-sic sub = U340ó Figure 4: Schematic cross-sectional view of the 4H-SiC SAM- APD, 1 p-layer, 2 n - -layer, 3 n - layer, 4 n - -layer, 5 n + - layer, 6 n-contact, 7 SiO 2, 8 filter, 9 p-contact licrofls Figure 5: 2-D distribution of electric filed at an avalanche breakdown voltage Fabrication of SiC APD arrays SiC APD arrays, as well as single APD devices, were fabricated using 3-inch diameter n-doped 4H-SiC substrates purchased from Cree Research (Durham, NC). The array APD devices had effective p-n junction diameters of 100 μm and with areas of UV light exposure of 44 μm in diameter. Single APD test devices varied from 200 to 500 μm. The APD structure consisted of a 0.2 μm n + cap layer (N d =2x10 18 cm -3 ), a 0.6 μm n - -layer (N d =2.0x10 16 cm -3 ), a 0.15 μm n - - layer (N d =6x10 17 cm -3 ), a 0.6 μm n - -layer (N d =1.4x10 16 cm -3 ), and a 2 μm p + -layer (N A =10 18 cm -3 ). ;;:: II.bwIsraTu Sbb[o flöopbb Ido'4LS :Qø,O Q o'ofl o-ki:. 250 μm 100 μm (a) (b) Figure 6: SEM images of (a) SiC APD array and (b) single test APD device with an optical filter. The processing steps used to fabricate the devices included mesa etching, passivation deposition, cathode and anode contact formation, and passivation removal in the active area. Reactive ion etching was used to define the mesas with the beveled geometry in order to eliminate edge breakdown effects. A thermally grown SiO 2 film was deposited to passivate the surface. Ni was sputtered to form the n-type contacts and a multiple-layer stack of Ti/Al/Ti/Ni was deposited to form the p-type contacts. Both contacts were annealed simultaneously at 1050ºC in a N 2 ambient. Finally, Ti/Au metal pads were deposited on top of the ohmic contacts and the passivation layer was removed in the active region by wet etching. In order to limit the response of SiC APDs to the solar-blind wavelengths, an optical filter was used to reject the light at wavelengths above 280 nm. A thin film optical filter comprised of HfO 2 and SiO 2 layers was used to provide both a packaged version and a version with the filter deposited directly on the SiC detector. The optical filter deposition was performed by Barr Associates Inc. [14] Inductively coupled plasma etching was used to open contact windows in the filter after its deposition on APD devices. Figure 6 shows SEM images of (a) SiC APD array and (b) single test APD device with an optical filter. Proc. of SPIE Vol J-6

7 3.3. SiC APD performance A typical dark current and gain characteristics for a group of devices from the best fabricated APDs are shown in Figure 7, while Figure 8 shows the spectral response of unfiltered 4H-SiC APDs measured at two different bias voltages in the wavelengths range from 200 to 400 nm. Current, A 1x10-5 1x10-6 1x10-7 1x10-8 1x10-9 1x Gain 10 5 dark current Voltage, V Figure 7: Dark current and gain characteristics of SiC APD Gain Responsivity, A/W V bias 50V bias 60% 50% Quantum Efficiency 40% 30% 20% 10% Wavelength, nm Figure 8: Spectral response of unfiltered 4H-SiC APDs measured at two different bias voltages in the wavelength range from 200 to 400 nm. The responsivity and external quantum efficiency measured at a reverse bias of 50 V are considered to be representative for the linear regime without any gain. At unity gain, the external quantum efficiency was 47% at 280 nm. With the bias increased to 100 V, the responsivity increase was likely caused by a slight increase in the external quantum efficiency as a result of modulation of the depletion width and an increase of the gain. Those two effects are very difficult to separate, but the external quantum efficiency did not exceed 57% at 280 nm. Several noticeable characteristic features affecting smoothness of the responsivity are attributed to the oxide layer left on the top surface of the sensitive area as an intermediate layer for the solar-blind filter stack deposition. The response starts to cut off at approximately 375 nm, corresponding to the bandgap of 4H-SiC. The fabricated SiC APDs did not show a measurable response to visible light at wavelengths greater than 400 nm. The long-wavelength cutoff is not sharp due to the indirect bandgap nature of SiC and thus an additional solar-blind optical filter was required to suppress responsivity in the 300 nm nm wavelength range in order to achieve a high solar photon rejection ratio in the near-uv range. Dark current density curves of a typical SiC APD 8x8 array are shown in Figure 9. It was possible to distinguish three characteristic groups (A, B and C) of the dark current curves for each array. Group A consists of APD devices with the lowest dark currents and very sharp avalanche breakdowns. Group B consists of APD devices with moderate leakage currents and less sharp avalanche breakdowns, while Group C consists of APD devices with high leakage currents and soft breakdowns. Note, that a value of a breakdown voltage for all devices for this array was 186±3 V at a current density of 10-4 A/cm 2. A histogram distribution of current densities measured at 95% of U BD is shown in Figure 10. One may see that the number of devices from Group A with a current density of ~10nA/cm 2 was 28 or 43 % per array of 64 devices, while numbers of devices from Groups B and C were 28 (43%) and 8 (14%), respectively. Proc. of SPIE Vol J-7

8 Current density, A/cm 2 1x10-4 1x10-5 1x10-6 1x10-7 1x10-8 1x10-9 C Voltage, V B A Figure 9: Dark current density curves of typical SiC APD 8x8 array. Current density, Log(A/cm 2 ) C B Number of devices A Figure 10: Distribution of current densities at 95% of a breakdown voltage in individual APDs in typical 8x8 array Using the Electron Beam Induced Current (EBIC) technique it has been found that such a variation in leakage currents is associated with different numbers of dislocations in SiC substrates that contained an individual APD device in the array. Moreover, the devices from Group A (with the lowest leakage currents) were found to be defect-free devices Single Photon Detection Efficiency The SPDE is an important characteristic of the avalanche photodiode and can be defined as the product of the quantum efficiency and the avalanche probability when operated at reverse biases above the breakdown voltage. Two passive quenching methods were used to measure single photon detection efficiency of devices without solar-blind filter. The first quasi-geiger method was utilizing simple asynchronous passive resistive quenching and the second method was using passive gated quenching. Figure 11 shows the schematic of the measurement setup. A 266 nm laser beam focused within the device with an average of 0.3 photons during a 500-ps pulse and an 8.3 khz repetition rate was used in both methods to illuminate the device. The breakdown voltage was measured to be around 194 V and the bias voltage for the quasi-geiger mode measurements was swept between 193 V and 200 V. The SPDE and dark count rate were measured for the same bias to obtain the curve shown in Figure 12. h R=50 Pulsed UV laser (266nm) V bias R=100K To Scope/ Counter Fiber optic delay line Beam splitter Focusing lens Photodiode T g = 9nsec h SiC APD Array Scope/ Counter C g =10nF R=50 V bias R=1M To Scope/ Counter Figure 11: Block diagram and schematics of the photon detection efficiency measurements in the Geiger mode: asynchronous passive quenching left, passive gated quenching right For gated passive quenching Geiger mode measurements, the dc bias voltage was set fixed at 190 V, or 4 V below the breakdown voltage. The excess ac-coupled voltage above the dc voltage was applied during a 9-ns gate pulse. The arrival of the photon was synchronized with the gate pulse. The avalanche pulses were discriminated and counted within a 1 nanosecond window. The excess voltage was scanned in the range from 5 to 30 V. Figure 13 shows the SPDE versus the dark count probability measured for the Proc. of SPIE Vol J-8

9 same over-bias. Three tested devices from different group (A, B and C) demonstrated different dark count rates and dark count probabilities. The difference in the dark characteristics can be explained by the presence of defects. In general, passive gated quenching demonstrated higher gains and better single photon detection efficiency, but ungated linear mode measurements have also demonstrated promising results. The simplicity of asynchronous passive quenching is one of the attractive advantages, although the detection efficiency is greatly reduced in devices with large number of dark counts. In such devices, the interference from dark counts affects sensitivity, suggesting that the passive gated quenching or asynchronous active quenching is a better quenching scheme to limit the amount of charge flown through the APD. Another complication arising from the operation of 4H-SiC APDs without fast quenching is the amount of after pulses initiated by the residual charge. Dark Count Rate, Hz A 10 1 B C Single Photon Detection Efficiency, % (266nm) Figure 12: Asynchronous resistive quenching dark count rate versus single photon detection efficiency at 266nm Dark Count Probability x10-4 1x A B C Single Photon Detection Efficiency, % (266nm) Figure 13: Passive gated quenching Geiger mode dark count probability versus single photon detection efficiency at 266nm. Separate absorption and multiplication 4H-SiC APDs have demonstrated 47% quantum efficiency at 280 nm wavelength. Single photon detection efficiencies of 32% and 37% were achieved at 266nm wavelength for passive quenching mode and gated Geiger-mode respectively. A dark count rate 10 khz and a corresponding dark count probability of 10-4 were recorded with the best device at a reverse bias providing the single photon detection efficiency more than 30% Actively-Quenched Geiger-Mode Operation with Time-of-Arrival Tagging Active quenching can provide the fastest reset times, and therefore the lowest blocking loss and highest achievable operational bandwidth for an array of GM-APDs. The active quenching circuitry for each APD can be combined with a precision clock reference and detection logic to enable each photon detection event to be time-tagged with a precision substantially better than 1 nanosecond. The combined quenching, detection, and time-tagging circuitry, termed a readout IC (ROIC), can be bump-bonded to an appropriately designed GM-APD array to form a compact integrated circuit for use in time-resolved photon-counting applications such as ladar [15]. In concert with the development of SiC GM- APD arrays by GE, MIT Lincoln Laboratory is fabricating a prototype 4x4 ROIC that will be wirebonded to a 4x4 APD array to demonstrate high-speed UV-C single photon detection and time tagging. A demonstration of the 4x4 prototype, scheduled for May of 2009, will provide quantitative performance metrics to guide the design of larger size arrays bump-bonded directly to a ROIC Solar-Blind filter Figure 14 shows the external quantum efficiencies of devices with and without near-uv blocking filters measured in the wavelengths range from 200 to 1000 nm at a reverse bias of 50 V. Devices with external and integrated solar blind filters showed rejection ratio of at least 10 6 for visible photons, but the rejection ratio of near-uv photons was slightly Proc. of SPIE Vol J-9

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